It is well known that the most efficient conversion of radiant energy to electrical energy with the least thermalization loss in semiconductor materials is accomplished by matching the photon energy of the incident radiation to the amount of energy needed to excite electrons in the semiconductor material to transcend the bandgap from the valence band to the conduction band. However, since solar radiation and blackbody radiation usually comprise a wide range of wavelengths, use of only one semiconductor material with one bandgap to absorb such radiant energy and convert it to electrical energy will result in large inefficiencies and energy losses to unwanted heat.
Ideally, there would be a semiconductor material with a bandgap to match the photon energy for every wavelength in the radiation. That kind of device is impractical, if not impossible, but persons skilled in the art are building monolithic stacks of different semiconductor materials into devices commonly called tandem converters and/or monolithic, multi-bandgap or multi-bandgap converters, to get two, three, four, or more bandgaps to match more closely to different wavelengths of radiation and, thereby, achieve more efficient conversion of radiant energy to electrical energy. Essentially, the radiation is directed first into a high bandgap semiconductor material, which absorbs the shorter wavelength, higher energy portions of the incident radiation and which is substantially transparent to longer wavelength, lower energy, portions of the incident radiation. Therefore, the higher energy portions of the radiant energy are converted to electric energy by the larger bandgap semiconductor materials without excessive thermalization and loss of energy in the form of heat, while the longer wavelength, lower energy portions of the radiation are transmitted to one or more subsequent semiconductor materials with smaller bandgaps for further selective absorption and conversion of remaining radiation to electrical energy.
Semiconductor compounds and alloys with bandgaps in the various desired energy ranges are known, but that knowledge alone does not solve the problem of making an efficient and useful energy conversion device. Defects in crystalline semiconductor materials, such as impurities, dislocations, and fractures provide unwanted recombination sites for photogenerated electron-hole pairs, resulting in decreased energy conversion efficiency. Therefore, high-performance, photovoltaic conversion cells comprising semiconductor materials with the desired bandgaps, often require high quality, epitaxially grown crystals with few, if any, defects. Growing the various structural layers of semiconductor materials required for a multi-bandgap, tandem, photovoltaic (PV) conversion device in a monolithic form is the most elegant, and possibly the most cost-effective, approach.
Epitaxial crystal growth of the various compound or alloy semiconductor layers with desired bandgaps is most successful, when all of the materials are lattice-matched (LM), so that semiconductor materials with larger crystal lattice constants are not interfaced with other materials that have smaller lattice constants or vice versa. Lattice-mismatching (LMM) in adjacent crystal materials causes lattice strain, which, when high enough, is usually manifested in dislocations, fractures, wafer bowing, and other problems that degrade or destroy electrical characteristics and capabilities of the device. Unfortunately, the semiconductor materials that have the desired bandgaps for absorption and conversion of radiant energy in some energy or wavelength bands do not always lattice match other semiconductor materials with other desired bandgaps for absorption and conversion of radiant energy in other energy or wavelength bands. Therefore, fabrication of device quality, multi-bandgap, monolithic, converter structures is difficult, if not impossible, for some portions of the radiation frequency or wavelength spectrum.
This problem has been particularly difficult to solve in the infrared (IR) portion of the spectrum, where options for suitable, commercially available substrates on which to grow thin films with the necessary bandgaps for absorption and conversion of the infrared radiation to electrical energy are very limited, and where compatible, i.e., lattice-matched, semiconductor materials with the different bandgaps needed to absorb and convert different portions of the infrared spectrum efficiently are also quite limited.
For example, the group III-V family of semiconductor alloys include some of the best materials for fabricating photovoltaic converters with bandgaps in a range of about 0.35 eV to 1.65 eV to absorb and convert infrared (IR) radiation with wavelengths in a range of about 3.54 μm to 0.75 μm. Group III-V alloys comprise combinations of binary compounds formed from Groups III and V of the Periodic Table. These binary compounds can be alloyed together into various ternary or quaternary compositions to obtain any desired bandgap in the range of 0.35 eV to 1.65 eV. These alloys also have direct bandgaps (i.e., no change in momentum is required for an electron to cross the bandgap between the valance band and the conduction band), which facilitate efficient absorption and conversion of radiant energy to electricity. However, InP, which has a lattice constant of 5.869 Å (sometimes rounded to 5.87 Å) and a bandgap of 1.35 eV, is one of only a few feasible, commercially available substrate materials with a lattice constant even close to those lower bandgap Group III-V alloys i.e., InP-based or related ternary and quaternary compounds. The lowest bandgap Group III-V alloy that can be lattice-matched to the 5.869 Å lattice constant of an InP substrate is Ga0.47In0.53As, which has a bandgap of about 0.74 eV, which leaves a significant range of lower frequency, longer wavelength (>1.67 μm), infrared (IR) radiation that cannot be absorbed and converted to electricity in monolithic converters in which the semiconductor absorption materials are lattice-matched to the substrate.
While the current unavailability of efficient and cost-effective solar photovoltaic (SPV) converters, especially multi-bandgap, monolithic, converter devices, capable of absorbing and converting infrared (IR) radiation in wavelengths greater than 1.67 μm leaves substantial amounts of energy in the solar spectrum to remain unconverted to electricity, in state-of-the-art SPV's, it is an even greater problem for thermophotovoltaic (TPV) devices. Infrared (IR) radiation of wavelengths greater than 1.67 μm comprises a substantial amount of the energy radiated from blackbodies, and thermophotovoltaic (TPV) converters are intended to absorb and convert as much radiant energy from blackbodies to electric power as possible. Therefore, solutions to these problems, especially if such solutions could enable fabrication of monolithic converters with multiple bandgaps in infrared (IR) energy ranges, they would facilitate capture of more electric energy from solar and/or blackbody radiation.
U.S. Pat. No. 5,479,032 issued to S. Forrest et al., teaches that one or more ternary InxGa1-xAs alloys with x>0.53, i.3., with band-gaps less than 0.75 eV, can be grown epitaxially on an InP substrate by using intervening, graded layers of InAsyP1-y between the InP substrate and the InxGa1-xP (x>0.53) layers. However, those Forrest et al., patent teachings, which were directed to pixel detection of near infrared radiation incident on a focal plane for telecommunications applications, are not useful in SPV and TPV applications.